1. Field of the Invention
The present invention relates generally to a magnetoresistive memory apparatus which records and regenerates magnetic information using a ferromagnetic tunnel junction (MTJ) device and, more particularly, to a magnetoresistive memory apparatus which can record magnetized information with magnetization inversion by small recording currents.
2. Description of the Related Arts
Traditionally, magnetoresistive memory (MRAM) has been attract attention as a memory apparatus which does not lose information after cutting the power supply and has no limitation on the number of reading and writing. Magnetoresistive memory is nonvolatile memory which has a degree of integration and high speed comparable to DRAM and can be rewritten without limitation, and is achieved by combining magnetic tunnel junction (MTJ) devices and MOS technology.
FIG. 1 is a circuit diagram of one (1) cell of magnetoresistive memory which consists of one (1) transistor and one (1) cell, and FIG. 2 shows a cross sectional view of a magnetoresistive memory cell. In a cell circuit of FIG. 1, a magnetoresistive memory cell consists of a magnetic tunnel junction device 100 and a MOS field-effect transistor 102; the magnetic tunnel junction device 100 is connected between a bit line 104 and a source S of the MOS field-effect transistor 102; a drain is connected to a plate line 106; and a gate G is connected to a read word line 108. In a cross section of the memory cell of FIG. 2, n channel layers 112 and 114, as well as a source electrode 116, a drain electrode 118 and the read word line 108 as a gate electrode are formed on a p channel semiconductor substrate 110.
As shown in FIG. 3, in the magnetic tunnel junction device 100, a ferromagnetic free layer 120 which has variable magnetization direction and ferromagnetic free layers 124 which has fixed magnetization direction is laminated via an insulating layer (Al—O layer) 122, and in a fixed magnetization layers 124, a anti-ferromagnetic pin layer 128 which is in exchange coupling with a ferromagnetic pinned layer 126 is laminated. To an inter-layer insulating film 130 formed on the semiconductor substrate 110 of FIG. 2, a plurality of bit lines 104 and read word lines 132 which intersect to constitute a matrix without contact are provided, and the magnetic tunnel junction device 100 which has a structure of FIG. 3 is located in a intersection between the bit lines 104 and the read word lines 132. This magnetic tunnel junction device 100 connects its free layer 120 side to the bit lines 104 and the pin layer 128 side to the source electrode 116 via an electric conductor 134. In such magnetoresistive memory, reading and writing are performed according to a following procedure.
(1) Writing
In the writing of magnetized information into the magnetic tunnel junction device 100 of the magnetoresistive memory, as shown in FIG. 4, by simultaneously sending currents Iy and Ix to the bit line 104 and the read word line 132 which are orthogonal above and under the magnetic tunnel junction device 100 and generating magnetic fields Hy and Hx, the magnetized information are written selectively. In this case, if the current is send to only one line of the bit line 104 and the read word line 132, the writing will not be performed. The free layer 120 acting as a write layer of the magnetic tunnel junction device 100, which is used as a memory device, is rectangular such that magnetic anisotropy (anisotropic magnetic field) is generated. In the magnetization direction of the free layer 120, the longitudinal direction of the rectangle is a stable direction, or an easy direction, because of its magnetic anisotropy. Therefore, magnetization in the stable direction is stable as long as an external magnetic field (switching magnetic field), which is necessary for reversal of the magnetization direction, is applied to it. Bits are set to “0” or “1” according to the magnetization direction in this free layer 120. As a method for selectively reversing the magnetization direction of the magnetic tunnel junction device 100, there is a method which applies the magnetic field Hy to a latitudinal direction, or a hard direction of the free layer 120 which is rectangular and simultaneously applies the magnetic field Hx to the easy direction. In other words, as shown in FIG. 4, by applying the magnetic field Hy to the hard direction of the free layer 120 with the write current Iy of the write word line 132, an energy barrier necessary for rotation of the magnetization direction may be lowered. At this point of time, if the magnetic field Hx is simultaneously applied to the easy direction of the free layer 126 with the write current Ix of the bit line 104, the magnetization direction of the free layer 120 of the magnetic tunnel junction device only, which is located in a current intersection, is turned to the easy direction which is the same as the magnetic field Hx.
FIG. 5 illustrates an asteroid curve which is thresholds of magnetic writing in the case that the easy directional magnetic field Hx and the hard directional magnetic field Hy are applied to the free layer. It is noted that the easy directional magnetic field Hx and the hard directional magnetic field Hy are normalized by the magnetic anisotropy (anisotropic magnetic field) Hk. This asteroid curve may be represented by following equation.Hx2/3+Hy2/3=Hk2/3The setting of a bit 0 or 1 according to the recording direction of the magnetized information is performed with combinations exceeding the thresholds given by the asteroid curve. For example, bit 1, 0 is set by recording a magnetization direction according to a combination of the easy directional magnetic field and the hard directional magnetic field of a point P1 located outside of the thresholds (Hx1/Hk, Hy1/Hk) and recording a turned magnetization direction according to a combination of the easy directional magnetic field and the hard directional magnetic field of a point P2 also located outside of the thresholds (−Hx1/Hk, Hy1/Hk).
(2) Reading
When the magnetic tunnel junction device 100 is selected by applying voltage to the bit line 104 and the read word line 108 of FIG. 1 and MOS field-effect transistor 102 is turned on, a current path through the magnetic tunnel junction device 100 is formed, and resistance is read. At this point, in the case that the magnetization direction of the free layer 120 of the magnetic tunnel junction device 100 is reversed by ferromagnetic tunnel effect, a resistance difference will be about 30 to 50%.
(3) Ferromagnetic Tunnel Effect
Generally, in a junction which has a “metal-insulator-metal” structure in semiconductor, when voltage is applied between the metals on both sides, if the insulator is substantially thin, an electric current flows slightly. Generally, an insulator does not conduct electricity, but if it has a thickness from several angstroms to several tens of angstroms which is substantially thin, it has probability to transmit just a few electrons due to quantum-mechanical effect, an electric current flows slightly. This current is called a “tunnel current”, and a junction which has this structure is called a “tunnel junction”. For the insulating layer 122 of the magnetic tunnel junction device 100, a metal oxide film is typically used as an insulating barrier. For example, a surface layer of aluminum is oxidized by natural oxidation, plasma oxidation and thermal oxidation. By adjusting oxidation conditions, it is possible to make several angstroms to several tens of angstroms of the surface into an oxide film. Because aluminum oxide is an insulator, it can be used as a barrier layer of the tunnel junction. A characteristic of these tunnel junctions is that, unlike normal resistance, an electric current for applied voltage has non-linearity, so this is used as nonlinear device.
A structure in which metals on both sides of the tunnel junction are replaced by ferromagnetic metals is called a magnetic tunnel junction. In the magnetic tunnel junction, it is known that the tunnel probability (tunnel resistance) depends on the magnetization conditions of magnetization layers on both sides. In other words, the tunnel resistance can be controlled by magnetic fields. Assuming that a relative angle of magnetization is θ, the tunnel resistance R can be represented by:R=Rs+0.5ΔR(1−cos θ)  (1)In other words, when angles of magnetization directions of the magnetization layers on both sides are the same (θ=0), the tunnel resistance becomes smaller(R=Rs), and when magnetization directions of the magnetization layers on both sides are opposite (θ=180), the tunnel resistance becomes larger (R=Rs+ΔR). This is caused by that electrons inside of the ferromagnetic material are polarized. Generally, in electrons, a up electron which is spinning upward and a down electron which is spinning downward are existed, and since the same numbers of both electrons are existed as electrons inside of normal nonmagnetic metals, it has no magnetization as a whole. On the other hand, in electrons inside of the ferromagnetic metals, the number of the up electrons (Nup) and the number of the down electrons (Ndown) are different, so it has magnetization depending on the up electrons or the down electrons. It is known that, if electrons tunnel, these electrons tunnel sustaining each spinning condition. Therefore, if there is vacancy in the electron condition of a destination of tunneling, tunneling is possible, but if there is no vacancy in the electron condition of a destination of tunneling, tunneling is not possible.
A change rate of the tunneling resistance is represented by a product of a polarization rate of a source of electrons and a polarization rate of a destination of tunneling.ΔR/Rs=2×P1×P2/(1−P1×P2)  (2)where P1 and P2 are polarization rates and represented by:P=2(Nup−Ndown)/(Nup+Ndown)  (3)The polarization rate P depends on kinds of the ferromagnetic metals. For example, polarization rates of NiFe, Co and CoFe are 0.3, 0.34 and 0.46, respectively, and in these cases, in a theoretical sense, about 20%, 26% and 54% of magnetoresistive change rates (MR ratios) are expected, respectively. These values of MR ratios are greater than anisotropic magnetoresistive effect (AMR) and giant magnetoresistive effect (GMR) and possible to apply to magnetic sensors and magnetic heads. Also, the tunnel resistance R depends on an insulating barrier height φ and a width W, according to next equation.R∞Exp (W×(φ)1/2)  (4)Therefore, the tunnel resistance R becomes small when the barrier height φ is low and the barrier width W is narrow.
FIG. 6 is a magnetoresistive effect curve (MR curve) of magnetoresistive tunnel junction which has such a spin-valve structure. Here, as a magnetoresistive tunnel junction of FIG. 7, if the structure consists of a pin layer 128 which is a anti-ferromagnetic layer, a pinned layer 126 which is a ferromagnetic layer, an insulating layer 122 and a free layer 120 which is a ferromagnetic layer, exchange coupling of CoFe layer which is the pinned layer 126 and Pt—Mn layer which is the pin layer 128 is formed, and the magnetization direction of the pinned layer 126 is fixed. Therefore, if a magnetic field is applied from outside, only the magnetization direction of the free layer 120, which is NiFe layer, is rotated. Then, because a relative angle of magnetization is changed between the free layer 120 and the pinned layer 126, the tunnel resistance R will be changed by resisting the magnetic field, as shown by equation (1) (See, e.g., Japan Patent Application Laid-open Pub. Nos. 2002-217382, 2002-299584 and 2002-367365).
By the way, if high-density memory is formed using magnetic tunnel junction devices, device sizes as well as wiring widths and pitches must be miniaturized. If the device size is miniaturized, holding power Hc of a free layer becomes greater, so a switching magnetic field applied to the free layer for reversing a magnetization direction must be increased. Increasing the switching magnetic field must involve increasing currents sent to a word line and a bit line for recording magnetized information. However, to increase currents, a size of a current drive circuit must be expanded, and this makes higher density difficult. Also, due to effects of both of the wiring miniaturization and the current increase, a density of currents increases and migrations are generated, resulting in problems such as disconnection.